Horsepower, torque, and gears

One way to understand horsepower and torque in your rig is that horsepower is how fast you can get up a grade and torque is what gear will be needed to do it. Both engine parameters are proportional to the engine speed from near idle up to where the engine is having too much trouble thrashing its pistons and such things.

Your engine will only go so slow and only so fast and it has a speed where it is at its most efficient. Engineers design vehicles so that the most efficient engine speed is used most, such as for cruising down the road but have to consider how the vehicle gets to that speed and how it can handle road conditions such as grades. Horsepower ratings are one of those things that get a lot of people going so there are all kinds of rules and regulations and arguing. There is some difference about what an engine will actually produce and what actually gets to the road. That is because there are losses between the engine and the tire and there are other things in the car that need engine power like lights, air conditioners, and other equipment.

The fact to keep in mind is that the range in horsepower (hp) for engines used on the road is rather small. Big 18 wheelers running 40 tons down the road may have a 500 hp engine while a small car in the next lane will have 100 hp for its 1 ton. Most tow vehicles have engines in the 200 to 400 horsepower range. That big rig is probably using the engine nearer its power maximums and that shows by its 6 miles per gallon fuel use. The small car isn’t taking all that much power from its smaller engine so it might get 40 miles per gallon. The lesson is that the power of the engine itself isn’t all that important compared to other things.

Most of the RV engines idle at about 700 RPM and cruise at about 2000 RPM. They may redline somewhere near 5000 RPM but you generally don’t want to run them above 4000, if that. Matching this engine speed to road speed is what gears and and torque is all about. The gears include those in your transmission and rear differential and also tire size. These are chosen to keep your engine in the optimum RPM ranges for normal travels. The transmission allows changing gear ratios so you can match the torque on the road to engine power in order to accelerate or climb hills. The gears act like a lever that can adapt the force and distance from the engine to a force needed to move the vehicle over a distance.

If you have a big and heavy rig it is like moving a big rock with a crow bar. The fulcrum or leverage point is close to the rock. You move your end of the bar a couple of feet and only move the rock a couple of inches. This is a lot of leverage and is the same idea as deep gears in a vehicle. That is why big rigs have lots of gears. A smaller vehicle doesn’t need the leverage so a change in its engine speed will be a fairly large change in road speed and it doesn’t need all the gears.
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See also:

Figuring how tire size influences distance traveled per rotation – Tire size calculator – the 235/85r16 for an E350 van (235 mm wide and .86 times that high in a 16 inch inside diameter) have about a 31.7″diameter or 99.7 inch circumference. A 235/75 has a circumference of 93.9 inches

Tire Size Vs. Gear Ratio – A Formula – rpm = mph x gear ratio x 336 / tire diameter

Truck engine performance questions answered at the Caterpillar On Highway engine FAQ

Torque vs Horsepower

Torque and power are inescapably linked by the fact that horsepower equals torque (in ft-pounds) times RPM divided by 5250, so people who talk as if they are independent are full of it. If you have a given torque curve for an engine, you have the horsepower curve also. Knowing how these two numbers work with each other lets you can poke through some of the BS you might read.

Torque and Horsepower by Gerald Luiz at Hummer at Off-Road.com – one of a number of ‘technical’ articles that can help you understand your vehicle.

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